1. Field of the Invention
This invention relates to electrical devices containing conductive polymer PTC elements, in particular self-limiting strip heaters, and their preparation.
2. Summary of the Prior Art
Self-limiting strip heaters are well-known. They comprise an element of a PTC conductive polymer having parallel electrodes embedded therein and an outer jacket of one or more layers of an insulating polymer surrounding the PTC element. Other electrical devices comprising a PTC element, one or more electrodes and an encapsulating jacket of an insulating polymer are also known. In the preparation of such devices, the device is often subjected to an annealing step in which it is heated to a temperature above the melting point of the PTC element in order to reduce the resistivity of the PTC composition. Reference may be made for example to U.S. Pat. Nos. 3,793,716, 3,823,217, (Kampe), 3,861,029 (Smith-Johannsen et al), 3,914,363 (Bedard et al) and 4,177,376 (Horsma et al) and to commonly assigned U.S. patent application Ser. Nos. 88,344 (Lutz) now abandoned 84,352 (Horsma et al), (MP0701), Ser. No. 134,354 732,792 (Van Konynenburg et al), now abandoned, 751,095 (Toy et al), now abandoned, 798,154 (Horsma), now abandoned 965,343 (Van Konynenburg et al), now U.S. Pat. No. 4,237,441, 965,344 (Middleman et al), now U.S. Pat. No. 4,238,812, 965,345 (Middleman et al) now abandoned and 75,413 (Van Konynenburg) and the eight applications filed Apr. 21, 1980 by Gotcher et al (MP0712, 157/111) Ser. No. 141,984, Middleman et al (MP0713, 157/112) Ser. No. 141,987, Fouts et al (MP0714, 157/113) Ser. No. 141,988, Evans (MP0715, 157/114) Ser. No. 141,989, Walty (MP0719, 157/161) Ser. No. 141,990, Fouts et al (MP0720, 157/162) Ser. No. 141,991, Middleman et al (MP0724, 157/167) Ser. No. 142,053 and Middleman et al (MP0725, 157/168) Ser. No. 142,054. The disclosure of each of these patents and applications is incorporated herein by reference.